Thermostatic switch having a permanent magnet for snap action closing



Oct. 12, 1965 T. E. LARSEN THERMOSTATIC SWITCH HAVING A PERMANENT MAGNET FOR SNAP ACTION CLOSING Filed March 26, 1962 INVENTOR. THEODORE E L425? BY \A ATTOPA/[Y United States Patent 3,211,864 THERMOSTATIC SWITCH HAVING A PERMA- NENT MAGNET FOR SNAP ACTION CLOSING Theodore E. Larsen, Minneapolis, Minn., assignor to Honeywell Inc., a corporation of Delaware Filed Mar. 26, 1962, Ser. No. 182,207 4 Claims. (Cl. 200-138) This invention is directed to an electric switch and particularly to an electrical switch formed by moving an electrically insulated magnet to magnetically engage a stationary magnetic member and move a resilient contact member positioned in-between against the stationary magnetic member to form a closed electric circuit.

In the drawing:

FIGURE 1 is an elevation, partly broken away, of a thermostatic switch,

FIGURE 2 is a fragmentary view of the switch mechanism taken from the right in FIGURE 1.

A thermostat shown in FIGURES 1 and 2 has a case 12 and a cover 14, both of sheet aluminum. A pair of mounting ears 15 are formed integrally with the case 12. A control point adjusting knob 17 located in front of the cover is connected to a pinion 19 which engages a gear sector 20 on a brass shaft 21 which is pivotally mounted in case 12 and cover 14. The inner end of a spiral bimetal element 24 is secured to the shaft 21. Knob 17 may be provided with suitable indicia to facilitate selection of the desired control point.

A stationary contact 26 is secured to the case 12 by an insulating block 27 and is provided with a terminal lug 28 outside the case. The contact 26 is made of soft steel and is silver plated to provide a good contact surface. A pair of movable contacts are carried on a contact arm in the form of a resilient blade 31 which is secured to case 12 by an insulating block 32 and is provided with a terminal lug 33 outside the case. The blade 31 normally holds the contacts 30 spaced slightly from stationary contact 26. An extension 31a on the upper end of the blade 31 is offset to the left and serves as an abutment engaged by a rectangular permanent magnet 35 fixed to a metal strip 37 which is carried on the outer end of spiral bimetal element 24.

The permanent magnet 35 is of rubber bonded barium ferrite in which the magnetic particles have been oriented in the manufacturing process. Before assembly in the thermostat, the magnet is saturated with the N and S poles arranged as shown in FIGURE 2. The degree of magnetization of this type of magnet can, however, be reduced after initial saturation. Use of this property is made in the described thermostat to adjust the force of the magnet after assembly. The magnet 35 cooperates magnetically with the stationary contact 26 and mechanically with the blade 31 to hold contacts 30 against contact 26 until suflicient force is built up in bimetal element 24, on temperature rise, to pull the magnet away from the stationary contact, and release the blade 31. On temperature fall, the magnet 35 will approach the stationary contact 26 until the magnetic attraction exceeds the restraining force of the bimetal element 24 and will then snap toward the stationary contact, engaging blade 31 and forcing contacts 30 against stationary contact 26. This will occur at a lower temperature than that at which the contacts separated on temperature rise and the difference between the two temperatures will be greater if the magnet 35 is more highly magnetized.

Since the bimetal element 24, together with the switch, are enclosed by case 12 and cover 14, the sensitivity of the thermostat depends principally upon conduction of heat between the aluminum case 12 and cover 14 and the bimetal element through the brass shaft 21 on which the element is mounted. It is, of course, desirable that the electrical circuit be isolated from the case and cover, and to this end insulation must be provided between the bimetal element and the switch blade 31, since insulation between the case or cover and the bimetal element would undesirably restrict the flow of heat. The necessary insulation is provided herein by the magnet 35 which is of the ceramic type and therefore a non-conductor. In addition, since the magnetic particles are embedded in rubber and the magnet formed by an extrusion process, the surface that engages the switch blade 31 is smooth and slightly compliant to assure consistant switch action with negligible wear of the abutting surfaces over a long service life, as well as quiet operation.

I claim:

1. In a thermostat, a frame, an electrical conducting member of magnetic material carried by said frame and electrically insulated therefrom, a resilient contact arm carried by said frame and electrically insulated therefrom with a contact portion thereof normally spaced to one side thereof but adapted on being flexed to engage said member, a bimetal element carried by said frame, and a permanent magnet of insulating material carried by said bimetal element on said one side of said conducting member and arranged to be moved by said bimetal element to cooperate magnetically with said conducting member while engaging and forcing said contact arm into engagement with said conducting member to close an electrical circuit therebetween.

2. In a thermostat, a case of good heat conducting material, an electrical conducting member carried by said case but electrically insulated therefrom, a resilient contact arm carried by said case but electrically insulated therefrom and having a contact portion normally spaced on one side from but adapted on being flexed to engage said member to form a closed electrical circuit, a bimetal element carried by said case in good heat conducting relation thereto, and a ceramic electrical insulating permanent magnet carried by said bimetal element and arranged on said one side of said member to be moved by said bimetal element to cooperate magnetically with said electrical conducting member while engaging and holding said contact arm in engagement with said conducting member.

3. In a thermostat, a frame, stationary contact means including a body of magnetic material carried by said frame, movable contact resiliently carried by said frame in spaced relation to said stationary contact means, a bimetal element carried by said frame, and a non-conducting permanent magnet carried by said bimetal, said magnet being positioned adjacent said contact means with said movable contact therebetween and adapted to be moved by said bimetal element to cooperate magnetically with said stationary contact means while engaging and 3 holding said movable. contact in flexed condition in engagement therewith.

4. In a thermostat, a case and cover of good heat conducting material, a bimetal element carried on a member of good heat conducting material extending between said case and said cover, stationary and movable contact means Within said case, said movable contact means being on one side of said stationary contact means, a permanent magnet of dielectric material carried on said bimetal, positioned on said one side and movable by said bimetal to engage said movable contact means, and a body of magnetic material carried by said case magnetically cooperating with said permanentmagnet to bias said magnet in a direction to actuate said contact, means.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 4/48 Great Britain.

BERNARD A. GILHEANY, Primary Examiner.

FEDERICK M. STRADER, Examiner. 

1. IN A THERMOSTAT, A FRAME, AN ELECTRICAL CONDUCTING MEMBER OF MAGNETIC MATERIAL CARRIED BY SAID FRAME AND ELECTRICALLY INSULATED THEREFROM, A RESILIENT CONTACT ARM CARRIED BY SAID FRAME AND ELECTRICALLY INSULATED THEREFROM WITH A CONTACT PORTION THEREOF NORMALLY SPACED TO ONE SIDE THEREOF BUT ADAPTED TO BEING FLEXED TO ENGAGE SAID MEMBER, A BIMETAL ELEMENT CARRIED BY SAID FRAME, AND A PERMANENT MAGNET OF INSULATING MATERIAL CARRIED BY SAID BIMETAL ELEMENT ON SAID ONE SIDE OF SAID CONDUCTING MEMBER AND ARRANGED TO BE MOVED BY SAID BIMETAL ELEMENT TO COOPERATE MAGNETICALLY WITH SAID CONDUCTING 